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A powerful earthquake struck central California on Monday, crushing two women to death in a cascade of rubble and causing high-rise buildings from San Francisco to Los Angeles to sway ominously.

In Paso Robles, the two women, aged 55 and 19, were killed when the town's landmark 19th-century clock tower building collapsed, spewing tonnes of debris on to the street below, officials said.

The San Luis Obispo County town, about 300 kilometres north of Los Angeles, was believed to be the hardest hit in the 6.5-magnitude quake that overall caused only modest damage, officials said.

Doug Monn, a city building official, said 82 buildings were damaged, many of them in the historic downtown area dating to the 1890s.

Nick Sherwin, the owner of Pan Jewellers, one of the stores in the clock tower building, said: "The building was just a rumble and a roar. I yelled to people to get out, and most did except for my wife and myself and an elderly couple. We threw ourselves over the elderly couple to protect them and when the rumbling stopped we all ran out."

First reports said three people were killed but officials said the two women were the only fatalities and another 46 people were injured, some with broken bones and lacerations. Rubble from the collapsing building hit parked cars.

State Assemblyman Abel Maldonado, who represents the Paso Robles area, said the two victims worked in a dress shop in the building and apparently ran outside when the quake struck, only to be killed by the masonry.

The US Geological Survey said initial damage reports were modest and could be measured in the millions of dollars. By contrast, the Northridge earthquake of 1994 in the Los Angeles area, of 6.7 magnitude, caused more than $US40 billion ($A54 billion) damage and ranks as one of the most expensive natural disasters in US history.

The quake, the largest to strike the seismically active state since 1999, cut power to more than 40,000 people but did not appear to cause the massive damage it might have, had it hit an area such as Los Angeles.

The state's power grid operator said there were no reports of damage to high-voltage lines and no damage to the Diablo Canyon nuclear power generator where the tremor was felt in the plant's control room.

The epicentre of Monday's quake was near San Simeon, the home of Hearst Castle, the mansion built by newspaper tycoon William Randolph Hearst.

The quake's depth along the San Simeon fault, part of the state's San Andreas fault system, was eight kilometres. The Geological Survey said the quake would produce hundreds of aftershocks over the next days, weeks or even years, but there was only a 5 to 10 per cent risk that they would be bigger than the initial quake.